1 Field of the Invention
The invention relates in general to the removal of contaminants from the waste streams generated in a waste incineration apparatus, particularly municipal solid waste (MSW) incinerator-type waste incineration apparatus and the like, by the use of a char.
2. Description of Related Art
The toxic effects of emissions of mercury, mercury compounds and high molecular weight organic compounds from incinerators operating on municipal, hazardous, or hospital waste has become a matter of increasing environmental concern. Mercury emissions have been limited by regulation to concentrations of between 10 xcexcg/Nm3 and 100 xcexcg/Nm3. Because of the volatility of mercury and its compounds formed in the incineration process and the toxic dioxins and furans formed in both the combustion and post combustion stages, their removal is problematic.
Attempts to remove mercury, mercury compounds and high molecular weight organic compounds from incinerator flue gas have been costly where significant efficiency is desired. As such, these toxins sometimes pass through conventional particulate collection devices, for example baghouses and electrostatic precipitators (with actual increase in dioxin concentrations in the electrostatic precipitator) and exit through effluent stacks in impermissibly high concentrations.
It has been known that a bed of activated carbon, in the presence of gaseous hydrogen chlorine, can adsorb and remove mercury vapor from the air passing thereover. Efforts to remove mercury and its compounds from flue gases using carbon have included attempts to enhance the adsorption capacity by pretreatment of the carbon adsorbent with hydrochloric acid gas. This method has generally been unsatisfactory because of the low degree of enhancement. Another approach has been to treat the carbon adsorbent with chloride salts. However this approach has also been, in general, unsatisfactory due to a low degree of enhancement, although Taiwanese Patent No. 055751 discloses a successful impregnated carbon, solid adsorbent.
Other attempts at reducing mercury emissions have included controlling baghouse temperature, providing semi-dry lime scrubbing systems with fabric filter collection, the addition of sodium sulfide to flue gas, and wet chemical scrubbing. U.S. Pat. No. 4,889,698 to Moller et al. describes the use of activated carbon to remove mercury from incinerator flue gases by injecting powdery activated carbon in the flue gas in connection with a spray absorption process. The cooling of the flue gases in the spray adsorption process to 90xc2x0 C.-180xc2x0 C. is taught to cause absorption of mercury and noxious polyorganic matter onto the powdery activated carbon.
It has also been known to add a carbonaceous material like coal to waste material being combusted. For instance, U.S. Pat. No. 4,886,000 to Holter et al. relates to a method of producing a sanitarily storable combustion product formed by the degasification of a mixture of garbage and high volatile coal. The coke-like pyrolysis product can be burned as fuel. U.S. Pat. No. 5,022,330 to Burgher et al. is directed to a garbage burning and melting apparatus wherein coal and garbage are mixed and placed in a blast furnace for burning with intense heat. Filters, e.g., smoke scrubbers, are disposed at the outlet of the furnace to remove various impurities from the flue gases exiting the blast furnace. However, these patents do not mention or relate to mercury removal and intend the complete combustion of the carbonaceous material and thereby avoid char formation.
The prior art methods of mercury removal discussed above all require costly treated carbon and/or additional steps, e.g., spray absorption or degasification to remove impurities. The foregoing demonstrates the need for a relatively simple, economic, efficient and effective method of removing mercury, mercury compounds and high molecular weight organics from solid waste incinerator processes and the like.
It is an object of the present invention to provide a method for controlling contaminant emissions from a waste incineration apparatus.
It is another object of the invention to reduce contaminants by the use of a char.
One or more of the above objects, as well as other objects of the invention, can be obtained by the present invention, the general method of which comprises separately supply a carbonaceous char to a flue gas within a waste incineration apparatus at a charring zone. The use of the carbonaceous char provides adsorption of one or more of the common contaminants contained in flue gas, if present, and thus insures that the flue gas complies with the targeted contaminant emissions level. One embodiment of the present method comprises burning contaminant-containing waste supplied to a waste incineration apparatus to form (i) ash and (ii) contaminant-containing flue gas, separately supply carbonaceous char at a charring zone of the waste incineration apparatus, and adsorbing a contaminant from the flue gas onto the char. In this embodiment, the carbonaceous char can be supplied as such or it can be supplied by introducing a carbonaceous material at the charring zone to thereby form a char in situ. In either event, the contamination level in the flue gas is reduced.
The present invention is based on the discovery that separately supplying char, produced in situ or added to the waste incineration apparatus, can cheaply, efficiently and effectively remove contaminants such as mercury or its compounds, from flue gas. Unlike some prior art strategies, which targeted the treatment of cooled flue gas after it has left the waste incineration apparatus, as is hereinafter defined, the present invention begins the contaminant removal process while the flue gas is still within the waste incineration apparatus. The char, which can be formed in the waste incineration apparatus or supplied thereto, preferably becomes entrained in the flue gas so that as the flue gas and char proceed through the waste incineration apparatus and into the particulate removal device, the char can continually collect the contaminant(s). Indeed, as long as the char is suspended in the flue gas, or is settled on a surface in contact with the flue gas, contaminant adsorption will continue on downstream of the waste incineration apparatus. This contaminant removal occurs without the need to carry out a post-waste incineration apparatus spray adsorption process. The entrained char can then be removed from the flue gas by the particulate removal device such as a baghouse.